Systems have been previously proposed which provide in-line mixing of a substance such as a fire retardant foaming agent or a fertiliser for example with water by flowing the water past a block containing the substance such that block dissolves or erodes to dose the water with the substance.
One of the problems with any such system is being able to control the rate of dosing of the substance in the water as it flows past the block, so that a generally constant composition of the mixture or solution is provided.